The present invention relates to the operation of a Delayed Coking Unit (DCU) comprising of feed heaters, main fractionator, wet gas compressor, and downstream light ends processing towers referred to as the Gas Plant. In particular, the present invention relates to determining when the process is deviating from normal operation and automatic generation of notification.
Delayed Coking is a high-severity thermal cracking process used in petroleum refineries. The process unit, DCU, thermally decomposes the “bottom” of the crude barrel, which are typically the bottom streams of the atmospheric and vacuum crude distillation towers and produces a value-added mixture of olefins, naphthas, gas oils and petroleum coke. The overall reaction is endothermic with the furnace supplying the necessary heat for vaporization and cracking. The olefins are used in the petrochemical industry. Naphthas are used for various gasoline blends. Gas Oils are sent to other refinery units to be further cracked into naphthas and olefins. The coke, which is essentially carbon with varying amounts of impurities, is calcined (roasted to dry, without melting) and used in the aluminum, steel or chemical industries. Coke can also be burned as fuel, or gasified to produced steam or electricity.
FIG. 23 shows a typical DCU layout. One or more fired heaters with horizontal tubes are used in the process to reach thermal cracking temperatures of 905 to 941° F. (485 to 505° C.). With short residence time in the furnace tubes, coking (formation of Petroleum Coke) of the feed material is “delayed” until it reaches a large drum downstream of the heater. The thermodynamic conditions of the drum are well-suited for the cracking operation to proceed. These drums are designed to normally operate at a top drum vapor temperature of 825° F. (441° C.) and a pressure of 15 psig (103 kpag). As the feed cracks, the cracked products (vapors) are sent into a fractionator while coke accumulates in the drum. The fractionator separates the hydrocarbon mixture received from the coke drum into various fractions. The overhead product of the fractionator is sent through wet gas compressors to a light ends processing unit to further separate the light mixture.
When the drum is filled mostly with coke, the feed from the furnace is directed to an empty drum. Multiple drums are thus operated in a staggered fashion to ensure continuity of operations of the furnaces, fractionator and the gas plant. The coke in the filled drum is quenched, cut and removed with high-pressure water to a pit located below the coke drums. A bridge crane is used to transfer coke from the pit to a pad where water is allowed to drain from the coke before it is crushed and loaded onto railcars for transport. The emptied drum is cleaned and readied for the next cycle. The furnaces are brought offline about once every 3 months to clean coke deposits formed over time in the tubes through a process known as “decoking”. In some refineries the furnaces are cleaned online through a process known as steam spalling. The delayed coking unit is thus capable of turndown to a nominal 50% of capacity which represents operation with one furnace and pair of drums out of service. The complete schematic with DCU and the downstream units is shown in FIG. 24.
Due to the complicated dynamic and semi-batch nature of the DCU, and due to the high-severity process conditions, abnormal process operations can easily result from various root problems that can escalate to serious problems and even cause plant shutdowns. Three problems typically plague the delayed coker units: 1) Premature coking of the heater tubes (instead of in the drum) resulting in reduced feed rates and reduced refinery throughput and eventual shutdown of the unit with significant economic losses; 2) Foam (produced while coking) carryover from the coke drum into the coker fractionator; 3) Reliability problems with the coker fractionator. These operations can have significant safety and economic implications ranging from lost production, equipment damage, environmental emissions, injuries and even death. A primary job of the operator is to identify the cause of the abnormal situation and execute compensatory or corrective actions in a timely and efficient manner.
The current commercial practice is to use advanced process control applications to automatically adjust the process in response to minor process disturbances, to rely on human process intervention for moderate to severe abnormal operations, and to use automatic emergency process shutdown systems for very severe abnormal operations. The normal practice to notify the console operator of the start of an abnormal process operation is through “process alarms”. These alarms are triggered when key process measurements (temperatures, pressures, flows, levels and compositions) violate predefined static set of operating ranges. These operating ranges are kept as wide as possible to avoid false alarms, and to avoid multiple related and repetitive alarms. Thus, when an alarm occurs, it is often too late for the operator to bring the process to normal operations without compromising the optimal production rates.
Furthermore, more than 600 key process measurements cover the operation of a typical DCU. Under the conventional Distributed Control System (DCS) system, the operator must survey this list of sensors and its trends, compare them with mental knowledge of normal DCU operation, and use their skill to discover the potential problems. Due to the very large number of sensors in an operating DCU, abnormalities can be and are easily missed. With the current DCS based monitoring technology, the only automated detection assistance an operator has is the DCS alarm system which is based on the alarming of each sensor when it violates predetermined limits. In any large-scale complex process such as the DCU, this type of notification is clearly a limitation as it often comes in too late for the operator to act to mitigate the problem. The present invention provides a more effective notification to the operator of the DCU.